1. Field of the Invention
The invention relates to a method for operating an extruder temperature controller with a stable temperature reset for an extrusion device. Specifically, the invention relates to a method for operating an extruder temperature controller to control the temperature of molten extrudate wherein the control alarm is delayed for a predetermined time when generating a control output driver signal to the heat exchange means at or near maximum capacity of an extruder system.
2. Description of the Background Art
Extrusion devices are often used in the plastics or other industries to continuously melt, blend, form, and solidify plastics or other materials into a desired shape. Typical extrusion devices include a rotating screw housed coaxially within a heated, cylindrically-shaped barrel. The screw rotates within the barrel and drives an extrusion material such as plastic through the barrel. The extrusion material is forced through a die or aperture at the end of the barrel. A temperature drop, that occurs when the extrusion material leaves the heated barrel, allows the material to solidify in a molded shape that is determined by the profile of the die.
The temperature of the extrusion material or plastic within the extruder barrel must be controlled so as to remain as near to a desired temperature as possible. An extruder barrel can be operated to control the temperature of the extrusion material within the barrel under one or more of three conditions. An extruder barrel can (1) add heat to a material, (2) extract heat from a material, or (3) maintain the heat of a material. The third condition of maintaining a temperature of an extrusion material occurs when an extruder is operated at a speed wherein the heat from the friction of the extrusion material created, as the material is processed in the extruder barrel, is approximately equal to the heat loss from the extruder barrel. This condition of no heat gain or loss in known as an “adiabatic” condition.
Most extrusion devices have a plurality of heat exchange zones. The temperature of each heat exchange zone can be independently controlled such that one or more heat exchange zones heat the material being processed while the remaining heat exchange zones are in an adiabatic condition or are cooling the extrusion material. It is common for a heat exchange zone near the end of an extruder barrel to be used to cool an extrusion material before the material is extruded through the die. This procedure allows the extrusion material to quickly solidify upon existing the die. An extruder barrel, typically, has eight heat exchange zones, but the number of zones can vary.
An extruder device can control the temperature of its extruder barrel with heat exchange elements. The extruder barrel is surrounded by a shell containing heat exchange elements. The heat exchange elements can be (1) heaters such as resistive heaters which increase the extruder barrel temperature and (2) cooling tubes for circulating water or another coolant in order to decrease the extruder barrel temperature. Alternative heat exchange elements can be used. For example, the cooling structure can be a finned shell with a blower that circulates air past the fins.
Temperature sensors, such as thermocouples, are positioned in extruder barrels to signal the temperature at the location of the sensor. Two thermocouples per barrel zone are usually provided and are electrically isolated from one another. A first thermocouple is known as the “A” thermocouple of the pair and is placed at the inner surface of the extruder barrel. A second thermocouple is known as the “B” thermocouple of the pair and is placed in the interior of the heater/cooler shell. Each zone of the extruder is similarly provided with a pair of thermocouples, A and B, similarly placed. An air-cooled extruder system also has the B thermocouple in the interior of the shell.
An extruder temperature controller receives signals from the temperature sensors. The extruder temperature controller determines whether the temperature of a given heat exchange zone is too cool or too hot and, if necessary, signals the appropriate heat exchange elements to increase or decrease the heat in the particular zone regulated by that controller.
The extruder barrel and the heat exchange elements are heat sinks and, thereby, cause a delay between the signalling of instructions by the extruder temperature controller to increase or decrease the temperature of a zone. For example, when the extruder temperature controller instructs a heating element to cease applying heat, energy stored in the heating element continues to warm that zone of the extruder barrel. This continued warming causes the extruder barrel temperature to continue to rise in that zone. The lag between the issuance of an instruction from the extruder temperature controller and the response from the heat exchange elements causes the extruder barrel temperature to oscillate about the desired temperature.
U.S. Pat. No. 3,866,669 to Gardiner and U.S. Pat. No. 3,751,014 to Waterloo both address the problem of oscillating extruder barrel temperatures. In the systems described in Gardiner and Waterloo patents, a first temperature probe or thermocouple provides a “deep” temperature measurement representative of the temperature of the extrusion material. A second thermocouple is positioned within the shell surrounding the extruder barrel to provide a “sl allow” temperature measurement representative of the temperature of the heat exchange elements. The electrical signals from the pair of thermocouples are combined to provide an average value. The extruder temperature controller monitors the average value and selectively activates the heating and cooling elements to maintain the average value at a temperature that is approximately equal to a setpoint representative of the desired temperature for the extrusion material.
The control of the heat exchange elements by an extruder temperature controller that is responsive to an average value for temperature rather than the actual temperature of the extrusion material, that is being processed, reduces temperature and/or control signal oscillations. An example of such a temperature oscillation occurs during operational conditions wherein a resistive heating element applies heat to increase the temperature of an extruder barrel. While the heating element is active, the shallow temperature measurement is higher than the deep temperature measurement. This temperature difference occurs because the shallow temperature probe is positioned in the vicinity of the activated heating element. Accordingly, the average value of the extruder temperature controller is also greater than the deep measurement or the actual temperature of the extrusion material. The average value reaches the temperature setpoint while the actual temperature of the extrusion material is still below the desired temperature. The extruder temperature controller inactivates the heating element after the average value reaches the temperature setpoint, but before the extrusion material reaches the desired temperature. The heat stored in the heating element continues to raise the temperature of the extrusion material toward the desired temperature. Such temperature oscillations can also occur during operational conditions wherein the temperature of the extrusion material is being decreased.
Inactivating the heat exchange elements before the extrusion material has reached the desired temperature prevents the temperature of the extrusion material from “overshooting” the desired temperature which can cause undesirable temperature oscillations. This advantage is achieved at the expense of a reduction in the accuracy with which the temperature of the extrusion material is controlled. More specifically, since the extruder temperature controller operates to correct the temperature only when the average temperature value deviates from the desired temperature, the extruder temperature controller may not attempt to adjust the temperature, even when the temperature of the extrusion material remains below a desired elevated temperature or above a desired cooling temperature.
U.S. Reissue Pat. No. Re. 31,903 to Faillace describes an extruder temperature controller which anticipates changes of temperature in an extruder barrel. This system monitors an average temperature value to determine when the temperature has not changed significantly for a specified length of time or when the system has “stabilized.” Once the system has stabilized, this extruder temperature controller examines the actual temperature of the extrusion material as indicated by the deep measurement and compares the actual temperature to the desired temperature. If the actual temperature is significantly different from the desired temperature, this extruder temperature controller calculates and changes the temperature setpoint so that the average value appears to require a temperature adjustment. If the actual extrusion material temperature is, for example, too low, the Faillace extruder temperature controller raises the setpoint above the desired temperature. The average value is then below the setpoint, which causes the extruder temperature controller to adjust the temperature until the average value is approximately equal to the temperature setpoint.
Changes in the rotational speed of the extruder screw or “screw speed” are normal during the start-up and the shutdown of an extrusion line. However, rotational screw speed changes typically cause a thermal load variation which is troublesome in an extrusion process. An example of this condition occurs in blow molding processes wherein the molded piece becomes jammed when existing the mold. Sensors, which detect the jammed piece, rapidly shutdown the extruder system in order to prevent further jams and potential damage to the mold system. The extruder system during normal operation in a blow molding process runs at a preset speed.
The extruder temperature controller of the Faillace Reissue Patent in a blow molding process resolves a reset value for each heat exchange zone. The reset value is proportional to the temperature offset for that heat exchange zone, which is proportional to the thermal load for that heat exchange zone. The Faillace extruder temperature controller resolves a reset value for each heat exchange zone individually.
When an extruder system for a blow molding process, using the controller of the Faillace Reissue Patent, is stopped due to a jam, it is typically restarted within a few minutes. The minimum time a heat exchange zone must be stable in control or “minimum reset stability time” is approximately four minutes. The actual time during which a heat exchange zone recovers from a step change in load, such as a sudden stop condition, is approximately 10 to 12 minutes. Therefore, the reset means in the Faillace extruder temperature controller cannot respond quickly enough to compensate for a step change in load which lasts for less than 10 to 12 minutes. The result of this condition is that a heat exchange zone is offset in temperature equal to the difference in thermal load at the normal running screw speed compared to the screw speed at stop. In addition, if the extruder system remains stopped for a period of time which allows the reset to actuate, such as when a jammed piece is cleared and the extruder system returns to a normal operating screw speed, the incorrect heat exchange zone temperature reset value causes a temperature offset. This temperature offset remains until a reset value can be resolved at the normal screw speed and compensates for the thermal load at that screw speed. This condition in a blow molding process causes a significant change in the characteristics of the plastic melt output of the extruder system. These changes cause a variation in the weight of the blow molded products. This variation can degrade the quality of the end product by causing variations in the wall thickness of the product. These variations in quality cause waste, inefficiency, and undue expense.
U.S. Pat. No. 5,149,193 to Faillace discloses an extruder temperature controller that preempts a temperature control set point for a heat exchange zone upon a change in the screw speed of the extruder system. This extruder temperature controller adjusts the control setpoint, in response to a change in the screw speed, which enables the controller to preempt an adverse change in the extruder barrel temperature and the temperature of the extrusion material in the barrel. The storing of a collection of previously calculated control setpoints for various screw speeds enables this extruder temperature controller to determine the appropriate control setpoint quickly by retrieving from memory the control setpoint corresponding to the current or actual screw speed. The previously calculated control setpoints enable an extruder system to avoid significant changes in temperature of the extrusion material or barrel temperature fluctuations both of which often accompany a search for a control setpoint to provide the desired barrel temperature.
The improved controller of the Faillace '193 patent permits heat exchange zone “reset value tables” to be entered for each profile. Upon selection of the profile number, the corresponding reset value tables are also selected. Also, the adaptive reset capability of this controller allows for deep and shallow temperature control with temperature reset to be applied to a plastic extrusion process where the screw speed of the extruder can change on a continual or unanticipated basis. This controller maintains the barrel temperature control, typically, within 1° F. of temperature stability at all operating screw speeds. The adaptive reset capability of this controller improves the plastic melt output of a extruder system during continual or unanticipated changes in operating screw speed. This capability greatly reduces the time to stabilize the heat exchange zone temperature control after a change in screw speed has occurred and improves product quality during start-up and shutdown of an extrusion process line and reduces scrap.
The extruder temperature controller of the Faillace '193 patent triggers a control alarm when the heating output reaches 100 percent. The control alarm resets the stability timer and a new reset is not calculated for the predetermined time of three or four minutes. This characteristic unnecessarily limits the extruder system from operating at or near 100 percent of its heating capability. This controller does not “learn” new reset values for screw speeds when the extruder barrel temperature is stable and does not clear stored screw speeds when a significant process change is detected.
The industry is lacking a method for operating a temperature controller for an extruder system with an adaptive reset capability and a dual sensor temperature controller that permits the extruder system to operate at or near its maximum heating capacity. Further, the industry lacks a method for operating a controller that learns new reset values for screw speeds when the extruder barrel temperature is stable and/or clears stored screw speeds when a significant process change is detected.